Additional Mailbox access issue

Hi

  We are having a strange problem today. One of the user is not able to access an additional mailbox though he has full access to it. User A has been given full permission to User Bs mailbox. However when we try to open user Bs mailbox from As outlook it says unable to expand the folder.

   1. We have exchange 2010 SP2 environment

   2. Outlook version 2003 and 2010.both doesnt work

   3. Tried using owa/userbaccount....It says your mailbox appears to be unavailable.

   4. Removed the full permission for user A and readded again. 

   5. Tried delegation from outlook. Still no success.

Any idea ? 

Thanks in advance